As a technology for a high-frequency power amplifier with a multi-stage configuration used in, for example, a mobile phone terminal or the like, the inventors of the present invention have examined the following technologies.
Recently, the mobile phones have rapidly spread and high-level services such as image communication also have become widely used. In order to respond to rapid increase in communication traffic amount, practical application of a third-generation system such as an EDGE system or W-CDMA system has started. The EDGE system is the third-generation system obtained by expanding communication speed of a GSM system which is a mainstream of the second-generation system and it can share the same frequency band with the GSM system, and thus, the spreading thereof in the future is expected.
A polar-loop transmitter is known as a transmitter configuration suitable for the EDGE system, and a configuration example thereof is shown in FIG. 2. The polar-loop transmitter includes an input terminal (IF input) 1, a phase detector (PD) 2, a VCO (voltage controlled oscillator) 3, a high-frequency power amplifier (PA) 4, a directional coupler (Coupler) 5, an output terminal (RF output) 6, an amplitude detector (AMD) 7, a variable gain amplifier (IVGA) 8, a down conversion mixer (DCM) 9, a variable gain amplifier (MVGA) 10, and a control terminal (Vramp) 11.
An IF input signal from the input terminal 1 is inputted into the phase detector 2 and the amplitude detector 7. The phase detector 2 outputs a voltage corresponding to a phase difference between the IF input signal and a feedback signal from the variable gain amplifier 10. An RF output from the VCO 3 is phase-modulated by an output from the phase detector 2. The amplitude detector 7 outputs a voltage corresponding to an amplitude difference between the IF input signal and a feedback signal from the variable gain amplifier 10. An output from the amplitude detector 7 is amplified by the variable gain amplifier 8. An output from the VCO 3 is amplified by the high-frequency power amplifier 4 and it is simultaneously amplitude-modulated by a signal from the variable gain amplifier 8. An output signal from the high-frequency power amplifier 4 is outputted to the output terminal 6 via the directional coupler 5. A part of the output signal from the high-frequency power amplifier 4 is branched at the directional coupler 5 to be inputted into the down conversion mixer 9 and then converted to an IF signal. An output from the down conversion mixer 9 is amplified by the variable gain amplifier 10 to be supplied as a feedback signal to the phase detector 2 and the amplitude detector 7. Gains in the variable gain amplifier 8 and the variable gain amplifier 10 are varied by a control signal from the control terminal 11.
In the polar-loop transmitter, linearity required for the EDGE system is satisfied by using a phase feedback loop configured of the phase detector 2, the VCO 3, the high-frequency power amplifier 4, the directional coupler 5, the down conversion mixer 9, and the variable gain amplifier 10 and an amplitude feedback loop configured of the amplitude detector 7, the variable gain amplifier 8, the high-frequency power amplifier 4, the directional coupler 5, the down conversion mixer 9, and the variable gain amplifier 10.
The high-frequency power amplifier 4 used in the polar-loop transmitter is a high-frequency power amplifier having a power control function, which has been conventionally used in the GSM system. However, a high-frequency power amplifier excellent in linearity of power control must be used in order to achieve the stable operation of the amplitude feedback loop.
As the technologies relating to such a high-frequency power amplifier, for example, the technologies described in Patent Documents 1 to 4 are known.
Japanese Patent Application Laid-Open Publication No. 2000-332542 (Patent Document 1) describes a bias circuit for a multi-stage power amplifier in which, in order to suppress increase of Rx noise at an output power reduction time in the multi-stage power amplifier configured of HBTs performing power amplification of a high-frequency signal, a signal Vapc inputted from an external control circuit is outputted only to a base of an HBT constituting an initial-stage amplifier in the power amplifier and a bias current from a constant voltage circuit whose operation is controlled by the signal Vapc is supplied to respective bases of the HBTs constituting the other amplifiers in the power amplifier.
Japanese Patent Application Laid-Open Publication No. 2004-40418 (Patent Document 2) describes a wireless machine in which a control voltage for varying transmission output supplied to a drive amplifier and a final amplifier is optimized so that the drive amplifier and the final amplifier always operate at their optimal operation points with respective transmission outputs. By this means, since the transmission efficiency can be improved, it is possible to achieve power saving and to widen the variable range of transmission output.